1. Field of the Invention
The present invention relates to a projection system in which imaging light emitted from a projector is projected on a projection screen to display thereon an image. More particularly, the present invention relates to a reflecting element that serves as a polarizer for changing the state of polarization of imaging light that is emitted from a projector, and to a projection system comprising the reflecting element.
2. Background Art
Conventional projection systems are usually as follows: imaging light emitted from a projector is projected on a projection screen, and viewers observe the light reflected from the projection screen as an image.
Typical examples of reflective projection screens for use in such conventional projection systems include white-colored paper or cloth materials, and plastic films coated with ink that scatters white light. Besides, higher-quality projection screens that comprise, so as to control scattering of imaging light, scattering layers containing beads, pearlescent pigments, or the like are now commercially available. All of these projection screens have no selectivity of polarization, so that, in projection systems comprising these projection screens, there has been no need to control the state of polarization of imaging light.
On the other hand, polarization screens using polarizers or polarized-light-separating elements are known as projection screens of a type different from the above-described ones (see Japanese Laid-Open Patent Publication No. 107660/1993). Such polarization screens have polarization selectivity with which they selectively reflect only light in a specified state of polarization. It is, therefore, possible to obtain excellent image quality (especially, contrast) even under bright environmental light if the state of polarization of imaging light is controlled so that the imaging light can be distinguished, by the state of polarization of the light, from external light such as interior illumination light.
Incidentally, in the case where a projector using a CRT (cathode ray tube) or DMD (digital micromirror device), for example, is used in a projection system comprising a polarization screen of the above-described type, since imaging light emitted from the CRT or DMD is unpolarized, it is necessary to place a polarization-controlling element, such as a polarizer for controlling the state of polarization of the imaging light, either in the vicinity of the aperture of the projector or on the optical path between the projector and the polarization screen, thereby converting the unpolarized imaging light into polarized one in a specified state of polarization. However, if a polarization-controlling element such as a polarizer is thus placed, unwanted polarized light is absorbed by the polarization-controlling element and is not used as imaging light. Therefore, the use of a polarization-controlling element has been disadvantageous in that the projector cannot exhibit its true performance and that the brightness of the imaging light is reduced to half.
On the other hand, when a liquid crystal projector is used as the projector, since it emits linearly polarized light under normal conditions, there can be used a polarization screen as it is if the polarization screen is of a type that reflects linearly polarized light. Further, even when a polarization screen of a type that reflects circularly polarized light or the like is used, it is possible to project, on the polarization screen, imaging light in the desired state of polarization without reducing the brightness of the imaging light if a quarter wave plate or the like is used.
However, when a liquid crystal projector is used as the projector, although the above-described reduction in the brightness of imaging light is not caused, the following problem occurs. Namely, in most commercially available liquid crystal projectors, the axes of polarization of light beams (linearly polarized light beams) in the respective wave ranges for the three primary colors (red (R), green (G), and blue (B)) making up imaging light are not identical (see Japanese Laid-Open Patent Publication No. 221449/2000), so that these projectors cannot be used as they are in projection systems comprising conventional polarization screens that reflect linearly polarized light beams whose axes of polarization are the same in all of the wave ranges. In projection systems comprising polarization screens that reflect circularly polarized light, linearly polarized light is used after converting it into circularly polarized light. Even in these projection systems, since the state of polarization of the linearly polarized light before conversion is not uniform, it is difficult to make the direction of optical rotation of the circularly polarized light after conversion (right-handed circularly polarized light and left-handed circularly polarized light) uniform.